1. Field of the Invention
The present invention relates to an image display device, and more particularly, to an active matrix organic electroluminescence display.
2. Description of the Related Art
There are great expectations on organic electroluminescence displays (hereinafter referred to as organic EL display devices) which each include an organic electroluminescence display panel (hereinafter referred to as organic EL display panel) driven by active matrix driving, as flat panel displays of a next generation.
The organic EL display panel usually includes an organic electroluminescence element (hereinafter referred to as organic EL element) and a driving-use thin film transistor for supplying a current to the organic EL element (hereinafter referred to as EL driver TFT).
As illustrated in FIG. 13, applying a constant current to the organic EL element lowers element's brightness (Br of FIG. 13) with time (T of FIG. 13), and the drop is accompanied by a rise in an anode voltage (Voled of FIG. 13) of the organic EL element. As illustrated in FIG. 14, a rate of this brightness deterioration (Brate of FIG. 14) and an increment value (Vdeg of FIG. 14) of the anode voltage (Voled of FIG. 14) have a linear relationship.
Consider a case where an image of a white quadrangle (square) as illustrated in FIG. 15 is kept displayed. A part in which the white square is displayed deteriorates more quickly than a part in which black is displayed, thereby creating a difference in brightness between adjacent pixels. When this brightness difference exceeds 1%, the incident is recognized as burn-in as illustrated in an area A of FIG. 15.
A diagram of FIG. 16 is obtained by scanning the anode voltage (Voled of FIG. 16) of organic EL elements along one display line (certain Y address) in an organic EL display panel that contains the place of burn-in in order of the elements' X addresses (Xadres of FIG. 16). A point A of FIG. 16 indicates a start point of the burn-in. A range B of FIG. 16 indicates a normal area, and a range C of FIG. 16 indicates the area deteriorated by the burn-in.
Conventional technologies of preventing burn-in are disclosed in JP 2005-156697 A, JP 2002-341825 A, and JP 2006-130824 A described below.
Technologies described in JP 2005-156697 A and JP 2002-341825 A enable an organic EL element to emit light stably without allowing burn-in by putting results of current measurement through A/D conversion and, based on resultant digital data, performing feedback control on an organic EL element driving voltage.
A technology described in JP 2006-130824 A corrects the organic EL element driving voltage by measuring a terminal voltage of an organic EL element and comparing the measured voltage against a default value. This technology corrects an organic EL element driving current based on a relation between the terminal voltage and current of the organic EL element which is recorded in advance.
Problems of the technologies described in JP 2005-156697 A, JP 2002-341825 A, and JP 2006-130824 A are as follows.
(1) JP 2005-156697 A and JP 2002-341825 A do not contain a concrete description on a signal fed back from the organic EL element to the EL driver TFT, and how a correction signal is generated is not clear. The technologies described in JP 2005-156697 A and JP 2002-341825 A therefore do not ensure precise correction even when accurate detection operation is carried out.
(2) The technology disclosed in JP 2006-130824 A which uses a pre-recorded relation between the terminal voltage and current of an organic EL element to thereby correct the driving current needs a data table of enormous size for the correction.